In the technology of field emission devices and structures, an electric potential applied to or near a pointed surface of an emission element or emitter (or a plurality of such emission elements or emitters configured in an array) stimulates the emission of electrons from the pointed surface. A shape of the emitting surface, e.g. a pointed emitter tip, is selected to concentrate the electric field formed by the potential and thus maximize electron emissions into a vacuum surrounding the emitter. Increasing the electrical field intensity increases a current density of the emitted electrons, and the intensity is inversely further related to a radius of curvature of the emitting surface shape. Extremely pointed field emission tips are therefore desired.
In a field emission display, electrons emitted from the emission element are accelerated in a vacuum to impinge a phosphor screen that glows when struck by the electron. By contrast, in a cathode ray tube display, the electrons are generated by thermal emission from a heated cathode surface. In the field emission display the electrons are emitted from a “cold” cathode surface.
As illustrated in FIG. 1, in a field emission display 6, electrons are generated by the field emission process from a cathode electrode 8 comprising an array of millions of sub-micrometer emission elements 10 formed within openings 11 in an insulator layer 12. Application of a voltage Vg between the cathode electrode 8 (overlying a cathode substrate 14) and a gate electrode 16 forms an electric field between the cathode electrode 8 and the gate electrode 16. The electric field causes the emission of electrons from the emission elements 10. In FIG. 1, the emitted electrons are represented by arrowheads 20.
A shape of the emission elements 10 is selected to maximize electron emission, as sharper emission elements produce more electrons and thus a brighter image. As the number of emission elements supplying electrons to each display pixel increases, the display reliability also increases, as it is known that the electron emissions from an emission element can decrease with time.
A voltage Va (greater than the voltage Vg) applied between the cathode electrode 8 and an anode electrode 24 accelerates the electrons toward a phosphor screen 25 (or other electroluminescent display device). The phosphor screen 25 and the anode electrode 24 are supported by a transparent anode substrate 26. Responsive to the impinging electrons, phosphor pixels comprising the phosphor screen 25 emit light observable from a surface 30 of the anode substrate 26. Typically, a plurality of emission elements 10 supply impinging electrons for a single pixel, wherein the plurality of emission elements 10 are insulated from other pluralities of emission elements 10, such that each plurality is independently controllable for emitting electrons that strike a single pixel.
For producing a color image, each pixel comprises a color pixel triad, further comprising a red sub-pixel, a green sub-pixel and a blue sub-pixel. The emission elements 10 associated with a pixel are segregated into a matrix of insulated addressable arrays, such that a first array is associated with the red sub-pixel, a second array is associated with the green sub-pixel and a third array is associated with the blue sub-pixel. To produce a blue color on the display, for example, the third emitter group is activated to emit electrons that impinge on the blue sub-pixel.
To permit operation at relatively low operating voltages, the emission elements 10 are typically constructed from a material exhibiting a low work function (such as molybdenum, where the work function is a measure of the amount of energy required for an electron to escape from the metal into the surrounding vacuum) to increase the electron emissions and shaped in the form of points 34. As can be seen from FIG. 1, the emission elements 10 (also referred to as cones) have a generally triangular shape with each emission element 10 pointed in a direction of the phosphor screen 25 such that electrons emitted from the emission elements 10 are directed toward the screen 25.
Application of the voltage Vg between the gate electrode 16 and the cathode electrode 8 controls emission of electrons from the emission elements 10. As can be seen in FIG. 1, the gate electrode 16 is disposed above the cathode electrode 8. To permit proper electron flow from the emitter emission elements 10 to the anode electrode 24, the openings 11 formed in the gate electrode 16 and the insulating layer 12 must be properly positioned with respect to the emission elements 10. A size and location of the openings affect not only the magnitude of electron flow from the emission elements 10, but also determine the shape and direction of the electron flux. The opening size and circumferential proximity to each emission element 10 determines the voltage Vg that is required for effective control of the electron emissions, while alignment of a hole axis with respect to an element axis controls the electron beam direction.
Opening/element alignment and opening size have been difficult to control in the prior art due to the extremely small geometries and tolerances associated with the openings 11 and the emission elements 10. Typically, to obtain opening/element alignment it has been necessary to employ a difficult and time-consuming masking step to form the openings 11, but slight errors in either the mask or the mask alignment relative to the substrate 14 can detrimentally affect the opening/element alignment and thus the emission of electrons. The difficulties encountered in fabricating such arrays increase significantly as the dimensions of the emitter emission elements 10 are reduced to a sub-micrometer or nanometer scale.
In addition to opening/element alignment concerns, according to the prior art the emission elements 10 are fabricated using known photolithographic masking, patterning and etching steps. This process limits element density and element quality. In particular, the density is limited by resolution of the photolithographic process. Also, since the emission elements are tapered, each occupies a larger area at a bottom surface than at a tip apex. Thus the required tapered base limits the emission element density, which lowers the image brightness. A higher element density is therefore desired to achieve a higher image brightness.
In an effort to overcome the disadvantages associated with the use of the photolithographic process for forming emitter emission elements, current research efforts form the emission elements 10 by directing a laser beam toward a substrate surface. When the laser beam strikes the surface material is removed therefrom, with the material remaining forming the emission elements 10. This process requires a laser scan over the entire substrate and thus can be time consuming. Disadvantageously, the emission elements 10 produced by the laser technique may not be uniform throughout the substrate.
Etching techniques to remove material layers from a silicon substrate are commonly used in semiconductor fabrication processes. Various dry and wet etchants are available, with each etchant offering specific etching characteristics, including material selectivity, etch uniformity and edge profile control. Plasma etching is one form of dry etching that employs a gas and plasma energy to create a chemical reaction that etches the desired material layer.
A conventional plasma etching system comprises a chamber, a vacuum system, a gas supply and a power source. After loading a silicon wafer onto a pedestal in the chamber, the vacuum system reduces the pressure and a reactive gas is supplied to the chamber. An electrode in the chamber is energized by a radio frequency power source to energize the gas to a plasma state, producing ions, electrons and radicals. A radio frequency bias applied to the substrate develops an electric field proximate the substrate to attract ions of the reactive gas to the substrate. These ions and the radicals synergistically etch the substrate according to a pattern in a mask overlying the substrate.
Selection of a specific reactive gas is based on the material to be removed during the etch process. For example, for etching a silicon dioxide material layer, CF4 and oxygen are typically used. In the energized state, the CF4 is disassociated into highly reactive carbon and fluorine radicals, in addition to a number of ions. The radicals and ions interact with the substrate, where the fluorine attacks the silicon dioxide, converting the silicon dioxide to a volatile material that is removed from the chamber by the vacuum system. Typically, the plasma etch process is performed at a temperature between about 15 and 45° C., and at a pressure between about 5 and 100 mTorr, depending on the reactor type employed for the process.